PHPBB has a decent security record. The issues that have been
discovered have been fixed in short order. It's when you don't apply
the latest patches and backup your database and installation directory
regularly that you run into problems.

If you don't follow basic principals of backups and updates, it
doesn't matter what software you run you'll have issues eventually.
It's amazing how many headaches tar, cron, and mysqldump can save you.

-- 
Andrew S. Zbikowski | http://andy.zibnet.us
 A password is like your underwear; Change it
 frequently, don't share it with others, and
     don't ask to borrow someone else's.